Solvent-based recovery and recycle of polyamide material

ABSTRACT

A process is disclosed for recovering polyamide material from post-industrial and post-consumer products containing the polyamide material and insoluble materials. The process includes the steps of: (a) contacting the post-industrial and post-consumer products with a suitable solvent in a reactor; (b) dissolving and partially depolymerizing the polyamide material in the solvent to form a solution by operating the reactor at a predetermined temperature and pressure and for a time sufficient to decrease the average molecular weight of the depolymerized polyamide to less than 90% of the initial molecular weight; (c) separating the insoluble material from the solution; and (d) recovering the depolymerized polyamide from the separated solution. The process may also include the step of repolymerizing the depolymerized polyamide.

FIELD OF THE INVENTION

[0001] This invention relates to a solvent-based process for therecovery and recycle of polyamide material from post-industrial andpost-consumer products. In particular, this invention allows theseparation of very small solid particles, such as TiO₂, from thepolyamide material to be recycled.

BACKGROUND OF THE INVENTION

[0002] Aliphatic polyamides, particularly nylon 6 and nylon 66, areextensively used in a variety of industrial and consumer products suchas carpets and automotive parts. In particular, carpets and automobileair bags contain large portions of polymers with a high polyamidecontent. Because of the great quantity of post-industrial andpost-consumer nylon made available each year, these nylon products areideal for recovery and recycle. Additionally, concerns over efficientresource utilization and environmental protection have created a needfor the recovery and recycle of nylon from discarded post-industrial andpost-consumer products.

[0003] Recycle processes are already used to recycle polyamide carpetwaste in order to minimize the portion of the polyamide-containingcarpet waste that has to be discarded. Mechanical means, such asgrinding and crushing, are known means for separation of solid polyamidematerial from foreign materials such as carpet backing, etc. Mechanicalseparation yields a low grade recycled product with limited uses. Inorder to produce a high-quality recycled polyamide product, the processmust remove impurities such as dyes, cotton thread, delusterants (TiO₂),dirt, and oil, among other things, that cannot be removed by mechanicalmeans alone.

[0004] There are several available non-mechanical approaches toreclamation of polyamide from discarded polyamide-containing products byisolating the polymer. Polyamides such as nylon are soluble in selectedsolvents, and thus solution-based processes offer routes to the recycleand recovery of polyamides. Suitable solvents are polar and oftenreactive with the nylon. Many need to be handled with extreme cautionfor safety reasons. From a processing point of view, ideal solventsshould have the following characteristics: environmental friendliness,cost-effectiveness, low toxicity, capability of dissolving polyamides atrelatively low temperatures, and capability of inducing polyamideprecipitation for subsequent separation from the solvent. As anadditional consideration in solvent-based recycle and recovery of nylon,a single solvent system rather than a mixture or a solution is generallydesired as a cost effective and easier to operate system. Certainpolyols and carboxylic acids have many attributes of ideal solvents forpolyamide recycle and recovery. However, polyols and carboxylic acidshave not been attractive solvents because they are reactive withpolyamides, and thereby have contributed to the degradation of molecularweight of the polyamide. In these prior art processes, slight losses inmolecular weight have been tolerated, however, it has been thought thatmore significant degradation is to be avoided because recovered degradedpolyamides are unsuitable for either extrusion purposes (e.g. fibers andfilms) or use as molding compounds.

[0005] U.S. Pat. No. 5430068 to Subramanian discloses a process torecover polyamides using anhydrous polyols or aliphatic carboxylic acidshaving from 2 to 6 carbon atoms as the solvent. The process alsoincludes the step of rapidly quenching the polyamide solution with anadditional quantity of solvent to avoid any significant degradation ofthe polyamide.

[0006] Other known solvent-based recycle and recovery processes useglycols as solvents, such as acetic acid and water, ethylene glycol andpropylene glycol. Glycol-based solvent processes take advantage of thedifferent solvencies of nylon 6 and nylon 66 at particular temperaturesto separate one from the other. However, glycols also react with thepolyamides, in this case to create higher molecular weight polyamides.Thus, the residence time, i.e. the time that the polyamide is contactedwith the solvent must be short to avoid glycol reaction with thepolyamide.

[0007] Aliphatic alcohols have been suggested for use as solvents inprocesses to recycle and recover polyamides. Methanol, in particular,has been shown to be useful in the separation of nylon 6 from nylon 66.Moreover, aliphatic alcohol solvents are effective under mildconditions, i.e. low temperature and short residence time. U.S. Pat. No.5840773 to Booij et al discloses a process to recover polyamides fromcarpet waste using an aliphatic alcohol as the solvent. The processconditions are such that “virtually no polyamide is decomposed” so thatdirectly reusable polyamides are obtained from the process.

[0008] U.S. Pat. No. 6036726 to Yang et al also discloses asolvent-based process to recover polyamides. Under the conditionsdescribed, the molecular weight of the recovered polyamide is“substantially unchanged”, making the polyamide readily available forre-use.

[0009] In these prior art processes, therefore, degradation anddepolymerization of the polyamide is avoided so that the recoveredpolyamide can be readily and directly reused to make finished products.One drawback, however, is that the viscosity of the unchanged polyamidesolution remains relatively high, making filtration with fine filtersdifficult. Smaller, finer particles, such as sub-micron TiO₂ particles,are difficult, if not impossible, to separate from the polyamidesolution. Thus, because the polyamide itself remains non-degraded in theexisting recycle and recovery processes, it is difficult to mechanicallyor chemically remove smaller impurities such as TiO₂, a delusterant,that may be part of post-industrial and post-consumer nylon products.

[0010] This problem is addressed by the present invention in which asolvent-based process is provided to recover polyamides frompost-industrial and post-consumer products. The process of the presentinvention allows for the separation of TiO₂ and other fine insolubleparticles by partially depolymerizing the polyamide to decrease itsviscosity.

SUMMARY OF THE INVENTION

[0011] Accordingly, in one aspect of the present invention, there isprovided a process for recovering polyamide material having an initialaverage molecular weight from post-industrial and post-consumer productscontaining the polyamide material and insoluble materials, the processcomprising the steps of:

[0012] (a) contacting the post-industrial and post-consumer productswith a suitable solvent in a reactor;

[0013] (b) dissolving and partially depolymerizing the polyamidematerial in the solvent to form a solution by operating the reactor at apredetermined temperature and pressure and for a time sufficient todecrease the average molecular weight of the depolymerized polyamide toless than 90% of the initial average molecular weight;

[0014] (c) separating the insoluble material from the solution; and

[0015] (d) recovering the depolymerized polyamide from the separatedsolution.

[0016] In a second aspect of the present invention, the process furthercomprises the step of repolymerizing the depolymerized polyamide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The preferred embodiment of the process of the present inventionis described with reference to the accompanying drawing in which:

[0018]FIG. 1 is a flow chart illustrating the steps in the preferredembodiment of the process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present invention will be described with reference to itspreferred embodiment.

[0020] The preferred process illustrated in FIG. 1 comprises 5 mainsteps, although it will be understood by persons skilled in the art thatStep 5 is a preferred step:

[0021] Step 1: Contacting the polyamide-containing post-industrial andpost-consumer product with a suitable solvent.

[0022] Step 2: Dissolving and partial depolymerizing the polyamide inthe solvent at a predetermined temperature and pressure for a timesufficient to decrease the average molecular weight of the polyamide toless that 90% of its original average molecular weight.

[0023] Step 3: Filtering the solution to remove solid impurities.

[0024] Step 4: Recovering the depolymerized polyamide from the solution.

[0025] Step 5: Repolymerizing the depolymerized polyamide so that it canbe used to make other useful products.

[0026] In Step 1, nylon-containing post-industrial and post-consumerproducts are collected.

[0027] The majority of recyclable nylon consists of nylon 6 and nylon 66formed by the homopolymerization of 6-aminocaproic acid, also known ase-caprolactam. Nylon 66 also known as nylon 6,6, is the polyamide formedby the reaction of adipic acid with hexamethylenediamine. It will beunderstood, however, that the process of the present invention is notlimited to nylon 6 and nylon 66, but also includes other polyamides suchas nylon 610 and nylon 612.

[0028] Waste and scrap nylon 6 and nylon 66 are available from manysources including but not limited to rejects, turnings and trimmingsfrom manufacturing processes, automotive parts, carpets, clothing, etc.The waste post-industrial and post-consumer products may be prepared forrecycling by any method that produces particulate material such asgrinding, crushing, etc. Alternately, if the source material is smallenough, it may be used whole. Nylon fiber may be used as is or may alsobe ground into smaller pieces. Nylon fiber in carpets may be separatedfrom the carpet backing, i.e. by shearing.

[0029] The nylon-containing products are then contacted with a suitablesolvent capable of dissolving and depolymerizing nylon underpredetermined conditions. The solvent used may be an aliphatic alcohol,such as ethanol or methanol. Preferably, methanol is used. The alcoholis preferably used in an anhydrous form, but it may also be in asolution of at least 90% by weight alcohol. Additionally, mixtures ofethanol, methanol or water may be used as the solvent, so long as thewater content is no more than 10% by weight.

[0030] In Step 2, the polyamide is dissolved and partially depolymerizedin the solvent in a reactor capable of operating at elevatedtemperatures and pressures. The temperature of the reactor is elevatedand pressure is increased to maintain a liquid phase. Sufficientresidence time is needed to allow sufficient depolymerization of thepolyamide to occur. For the depolymerization of nylon 6 and nylon 66,the preferred temperature range is 160 to 210° C.; the preferredpressure range is 350 to 600 psig; and the residence time necessary fordepolymerization ranges from about 30 to about 400 minutes. Under thesecontrolled conditions, depolymerization of the polyamide reduces theaverage molecular weight of the polyamide from between about 10% toabout 75%. This results in a decrease in the viscosity of the solution.

[0031] In Step 3, the solution containing the dissolved anddepolymerized nylon and insoluble materials is passed through suitablefiltration media to remove and separate the insoluble materials from thesolution. Any suitable method of filtration maybe used. The preferredfiltration method involves passing the dissolved and depolymerized nylonsolution through glass fibers. In this preferred method, glass woolfiltration involves passing the solution, under pressure of about 500psig, through glass wool supported by wire mesh. Throughout thisfiltration step, all operating conditions including temperature,pressure and solvent concentration are maintained within theabove-described ranges to keep the dissolved and depolymerized nylon insolution.

[0032] This filtration step separates insoluble material includingsub-micron sized particulate matter such as TiO₂, from the solution.This results in a substantially more purified, low viscosity solutioncontaining the depolymerized nylon. Because the nylon is not onlydissolved but also depolymerized, much finer solid particles can beremoved than would otherwise be possible with a solution containinglarge, intact higher viscosity nylon polymer. Finer filtration makespossible the removal of small sized impurities such as TiO₂ that arecommonly found in nylon-containing products. In addition, because thenylon to be recyled is depolymerized, greater concentrations of nylon insolvent and longer residence times in the reactor are possible whilestill allowing for an acceptable recovered product. This is because thedepolymerized nylon has a lower viscosity than that of the original,dissolved polymerized nylon.

[0033] In Step 4, the depolymerized nylon may be recovered by anyrecovery method known to persons skilled in the art, such as precipationof the depolymerized polyamides, spray drying, and flash evaporation.Preferably, the depolymerized nylon is precipitated out and removed fromthe filtered solution of Step 3 by either cooling the solution ordiluting the solvent with an additive that forces the nylon out ofsolution, such as an anti-solvent agent. Once out of solution, thedepolymerized nylon is separated from the solvent by filtration orcentrifugation. Residual solvent must be removed from the depolymerizednylon before repolymerization. Removing nylon from the solvent in Step 4produces both a reduced molecular weight (depolymerized) nylon and asolvent with soluble impurities therein. The solvent may be purified byexisting technology and recycled back to Step 1 at the beginning of thisprocess. Repolymerization of the recovered polyamide is the finaloptional step in the process. Repolymerization begins withlow-viscosity, low molecular weight polyamide and results in a higherviscosity, higher quality polyamide suitable for end-use processing. Therepolymerization of nylon can be done readily by one of two standardmethods: solid-phase repolymerization or melt-phase repolymerization.The choice of repolymerization method depends on the need for greaterresidence time, i.e. the time needed for repolymerization to occur.

[0034] The solid-phase method of repolymerizing nylon is carried out atany suitable temperature below the melting point of the polyamide. ForNylon 6, the upper temperature limit is 220° C. whereas for Nylon 66 itis 265° C. The preferred temperature range for solid-phaserepolymerization is 160-200° C. Solid-phase repolymerization has noresidence time limit, i.e. the repolymerization process may take as longas is needed, and therefore is useful for recovery of significantlydepolymerized polyamides. Solid-phase repolymerization yields a highquality, high molecular weight polyamide.

[0035] In the melt-phase repolymerization method, the repolymerizationoccurs at a temperature above the melting temperature of the polyamide;the preferred temperature range of operation is 270-300° C. and thepreferred residence time limit for repolymerization is 1-30 minutes.Melt-phase repolymerization is a faster, simpler process compared tosolid-phase repolymerization and is useful if the final product to bemade from the repolymerized polyamide is an extruded product.

[0036] Once repolymerization is complete, the recycled and recoveredpolyamide can be manufactured into nylon-containing industrial andconsumer products.

EXAMPLES EXAMPLES 1

[0037] Nylon fibers were mechanically separated from carpet waste toproduce a mixture containing 92% Nylon 66. The other 8% was a mixture oflatex and polypropylene. 150 g of the separated Nylon fibers was thenadded to a one gallon stainless steel reactor equipped with an agitator.The reactor had a drain line on the bottom that was equipped with ashut-off valve and a metering valve. Prior to adding the nylon andsolvent to the reactor, a wire mesh screen (80 mesh) and glass filters(Whatman type GF/D and GF/B) were placed on the bottom of the reactorand a metal ring was inserted to hold the filters in position.Approximately ½ inch of glass wool was placed on top of the filters.After adding the nylon to the top of the reactor system, the reactor wassealed and heated to 185° C. Two liters of methanol were pumped at arate of 110 ml/min through a preheater and into the reactor. The systemwas then agitated and held at an operating temperature of 185° C. for 30minutes. The shutoff valve was then opened on the drain line allowingthe solution to flow through the filter system. The filtered solutionflowed through the metering valve and into a product collection tankfilled with an ice/water mixture, causing the nylon to precipitate. Theprecipitated nylon was separated from the solution with a wire meshscreen and removed to dry in a vacuum oven. The nylon product wasanalyzed by hydrolysis followed by gas chromatography for nylon content;molecular weight was determined by relative viscosity in 85% formicacid. The results are summarized in Table 1.

[0038] In this specification, relative viscosity is defined as the ratioof the viscosity of an 8.4% (by weight) solution of the polymerdissolved in 85% formic acid to the absolute viscosity of the 85% formicacid used. Polyamide from the product was weighed and dissolved in 85%formic acid. The viscosity was determined by automated measurement ofthe time required for the sample to flow through a calibrated Ubbelohdeviscometer (drop time).

[0039] Methyl group occurrence per 106 gram of polyamide was used as anestimate of the occurrence of reaction between the solvent and thepolymer. This shows the extent to which the solvent reacted with thepolymer. This was measured by proton nuclear magnetic resonance.

EXAMPLE 2

[0040] Example 2 was run at the same conditions as Example I except thatthe hold time in the reactor at operating conditions after solventaddition was 60 minutes. The results are summarized in Table I

EXAMPLE 3

[0041] Example 3 was run at the same conditions as Example 1 except thatthe hold time in the reactor at operating conditions after solventaddition was 120 minutes. The results are summarized in Table 1.

EXAMPLE 4

[0042] Example 4 was run at the same conditions as Example 1 except 100g of Nylon was initially added to the reactor and the hold time in thereactor after solvent addition was 180 minutes. The results aresummarized in Table 1.

EXAMPLE 5

[0043] Example 5 was run at the same conditions as Example 1 except 100g of Nylon was initially added to the reactor, the operating temperaturewas 175° C., and the hold time in the reactor after solvent addition was120 minutes. The results are summarized in Table 1.

EXAMPLE 6

[0044] Example 6 was run at the same conditions as Example 1 except theproduct collected was placed in an oven that was nitrogen sparged for170 minutes at 190° C. The results are summarized in Table 1.

EXAMPLE 7

[0045] Example 7 was run at the same conditions as Example 1 mexcept theproduct collected was placed in an oven that was nitrogen sparged for340 minutes at 190° C. The results are summarized in Table 1. TABLE 1Sample Relative Viscosity Methyl (eq/106g) Feedstock 50 0 Example 1 3253 Example 2 27 72 Example 3 20 85 Example 4 19 111 Example 5 21 111Example 6 60 24 Example 7 72 34

EXAMPLE 8

[0046] Example 8 was run using conditions similar to Example 1. Thereactor system was modified with a second pressurized vessel for productcollection. Once the solution was passed through the filters, it wascollected in the second vessel and allowed to cool. Cooling wasaccomplished by flowing water through a cooling coil installed in thevessel. After cooling the precipitated polymer was separated from thesolvent. Residual solvent was removed from the precipitated polymer bydrying in a vacuum oven. The charge to the reactor was 200 grams and theoperating temperature was 170° C. The hold time in the reactor aftersolvent addition was 30 minutes. The final product was analyzed fortitanium dioxide content by neutron activation analysis. The results aresummarized in Table 2 below.

EXAMPLE 9

[0047] EXAMPLE 9 was run under conditions identical to Example 8 exceptthat the hold time in the reactor after solvent addition was 60 minutes.The results are also summarized in Table 2.

EXAMPLE 10

[0048] Example 10 was run under conditions identical to Example 8 exceptthat the hold time in the reactor after solvent addition was 90 minutes.The results are also summarized in Table 2. TABLE 2 Sample Ti (neutronanalysis) Feedstock 1400 ppm Example 8 210 ppm Example 9 145 ppm Example10 46 ppm

[0049] Although the present invention has been shown and described withrespect to its preferred embodiments, it will be understood by thoseskilled in the art that other changes, modifications, additions andomissions may be made without departing from the substance and the scopeof the present invention as defined by the attached claims.

What is claimed is:
 1. A process for recovering polyamide materialhaving an initial average molecular weight from post-industrial andpost-consumer products containing the polyamide material and insolublematerials, the process comprising the steps of: (a) contacting thepost-industrial and post-consumer products with a suitable solvent in areactor; (b) dissolving and partially depolymerizing the polyamidematerial in the solvent to form a solution by operating the reactor at apredetermined temperature and pressure and for a time sufficient todecrease the average molecular weight of the depolymerized polyamide toless than 90% of the initial average molecular weight; (c) separatingthe insoluble material from the solution; and (d) recovering thedepolymerized polyamide from the separated solution.
 2. The process ofclaim 1, further comprising the step of repolymerizing the depolymerizedpolyamide.
 3. The process of claim 2, wherein the repolymerizedpolyamide has an average molecular weight substantially the same as theinitial average molecular weight.
 4. The process of claim 1, wherein thesolvent is an aliphatic alcohol.
 5. The process of claim 4, wherein thesolvent is anhydrous methanol.
 6. The process of claim 4, wherein thesolvent is a solution containing at least 90% methanol.
 7. The processof claim 4, wherein the solvent is anhydrous ethanol.
 8. The process ofclaim 4, wherein the solvent is a solution containing at least 90%ethanol.
 9. The process of claim 1, wherein the reactor is operated at atemperature between 160° C. and 210° C. and a pressure sufficient tokeep the solvent in liquid phase for 30 to 400 minutes.
 10. The processof claim 9, wherein the pressure is at least 350 psig.
 11. The processof claim 2, wherein the repolymerization occurs through a solid-phaserepolymerization process at a temperature between 160° C. and thetemperature at which the polyamide melts.
 12. The process of claim 2,wherein the repolymerization occurs through a melt-phaserepolymerization process at a temperature above the melting temperatureof the polyamide.
 13. The process of claim 1, wherein the insolublematerial is separated by passing the solution through suitablefiltration means.
 14. The process of claim 13, wherein the filtrationmeans is selected from the group consisting of glass fibers, insolublematerial residue and a combination of these.
 15. The process of claim14, wherein the filtration means comprises insoluble material residuethat has accumulated on the filtration means.
 16. The process of claim14, wherein the filtration media is glass fibers.
 17. The process ofclaim 1, wherein the depolymerized polyamide is recovered by a methodselected from the group consisting of precipation of the depolymerizedpolyamides, spray drying, and flash evaporation.
 18. The process ofclaim 17, wherein the depolymerized polyamide is recovered by coolingthe separated solution to a temperature sufficient to causeprecipitation of the depolymerized polyamides.
 19. The process of claim17, wherein the depolymerized polyamide is recovered by adding to theseparated solution an anti-solvent agent to cause precipitation of thedepolymerized polyamides.
 20. The process of claim 18 or claim 19,further comprising the step of purifying the separated solution afterthe depolymerized polyamide is recovered, and using the purifiedsolution as the solvent in step (a).
 21. The process of claim 1, whereinthe post-industrial and post-consumer products include one or more ofcarpet waste, glass-reinforced nylon, air-intake manifolds, radiatorend-caps, coated fabrics, air bag fabrics and mineral-filled nylon. 22.The process of claim 1, wherein the insoluble material includes TiO₂.